The invention relates to a flow control valve, and in particular, to such valve which exhibits a non-return drooping response, that is, providing a flow of a magnitude fed to a hydraulic instrument which decreases with an increase in a discharge flow from a pump and which is free from the influence of a pressure fluctuation in the hydraulic instrument upon the flow being fed to cause the latter to return.
A flow control valve which exhibits a response as described above is useful in providing a vehicle stability when running at high speeds and a reduction in the horsepower dissipated when used in a power steering apparatus associated with a vehicle. In conventional arrangements of this kind, an orifice is formed in a passage which supplies hydraulic fluid discharged from a pump to a hydraulic instrument, and a pressure differential across the orifice is utilized to open a spool valve to return part of the hydraulic fluid while simultaneously providing a restriction in the passage so that the orifice may be reduced by a control spool which is responsive to a pressure differential across the restriction (see Japanese Laid-Open patent application Nos. 104,186/1981 and 4,469/1982).
However, conventional flow control valves are complex in construction and require an increased number of parts, which in turn require a high accuracy in their manufacturing. In addition, a tuning of the response is difficult to achieve.
To overcome such difficulty, the present inventors have previously proposed a flow control valve having a greatly simplified construction and exhibiting a drooping response (U.S. Ser. No. 776,556, now U.S. Pat. No. 4,700,733). In this arrangement, a cylindrical union is fixedly mounted in the opening of a bore which receives a spool valve therein, and is formed with an orifice which is effective to open the spool valve in response to a pressure differential thereacross. A sleeve is fitted around the union to define a restricted passage over the outer surface of the sleeve which is effective to restrict the passage of a hydraulic fluid. A pressure difference between the upstream and the downstream side of the restricted passage causes the sleeve to slide, thus reducing the size of the orifice.
However, while successfully achieving the objective of providing a non-return drooping response with a simplified construction, the flow control valve described in the immediately preceding paragraph suffers from a limited degree of freedom in the choice of the response because the restricted passage is defined between the bore receiving the spool valve and the outer surface of the sleeve to present a constant flow area of the passage, whereby the pressure differential across the sleeve is determined only by the flow rate through the restricted passage. In addition, a retainer ring is used as one of seats against which a spring urging the sleeve abuts, leaving a certain space for improvement in respect of the number of required machining steps and the number of parts used.